1. Field of the Invention
This invention relates generally to a key seal and a seal assembly in which a key seal may be used, and, more particularly, to a seal having a modified cross section which may be used in a bi-directional seal assembly.
2. Background of the Invention
Bi-directional seal assemblies function when pressure acts on them from two different directions. Such bi-directional seal assemblies are used in various different types of valves. For example, bi-directional seal assemblies are disclosed in U.S. Pat. Nos. 7,073,590 and 6,702,024. Such a prior art bi-directional seal assembly 36 is used in the dirty fluid valve 10 shown in FIGS. 1 and 2. Valve 10 may be positioned in a downhole tool that is used for sampling wellbore fluids. When the tool is inserted in the wellbore, the valve 10 is in the closed position as shown in FIG. 1. When the downhole tool reaches a depth that needs to be sampled, a pilot valve is pulsed, causing a seal carrier to slide the bi-directional seal assembly 36 along opposing seal plates 42, 50 so as to open the supply ports 46, 54, as shown in FIG. 2. This allows wellbore fluids to enter the supply ports 46, 54 of the dirty fluid valve 10 and move through the longitudinal passageway of the valve 10 and out the function ports 56, 58 to a sample collection bottle.
External pressures in a wellbore often exceed 20,000 psi absolute. After a sample has been collected, a pilot valve is pulsed, causing the seal carrier to move back to the close position as shown in FIG. 1. The pressure inside the sample collection bottle is the same as the pressure in the wellbore at the collection depth. As the downhole tool is brought back to the surface, external pressure drops to standard atmospheric pressure, but the pressure inside the sample collection bottle remains at wellbore pressure, which may be in excess of 20,000 psi absolute. Bi-directional seal assembly 36 must therefore function when pressure acts on it from below, as well as from supply ports 46, 54.
FIG. 3 illustrates a cross-section of a prior art bi-directional seal assembly 136. This prior art seal assembly 136 includes a seal spool 164, first and second O-rings 166, 168, and first and second seal cups 160, 162. Four back-up rings 301, 302, 303, 304, are also shown. Although optional, these back-up rings generally help to prevent extrusion at higher pressures. First and second O-rings 166, 168 are installed around opposite ends 204, 206 of axle 200, with central collar 202 residing therebetween. First seal cup 160 engages with the first end 204 of axle 200, while the second seal cup 162 engages with the second end 206 of axle 200.
In operation, in the open position shown in FIG. 2, pressure coming from supply ports 46, 54, forces O-rings 166, 168 radially outward, somewhat deforming them into a shape with a generally oval cross-section. In this position, O-rings 166, 168 force seal cups 160, 162 outwardly against seal plates 42, 50, thereby creating a strong metal-to-metal seal. Once a sample has been collected and the valve 10 has shifted back into its closed position as shown in FIG. 1, the samples collected remain at their very high collection pressures. As the valve 10 and sample bottles are brought back to the surface, these high pressure samples exert pressure on the bi-directional seal assembly 36 from a different direction. This function pressure comes from function ports 56, 58, and passes up through longitudinal bore 14. The function pressure then forces O-rings 166, 168 radially inward, again somewhat deforming them into shapes with oval cross-sections. In this position, O-rings 166, 168 force seal cups 160, 162 outwardly against seal plates 42, 50, thereby creating a strong metal-to-metal seal.
Preferably, the O-rings 166, 168 are squeezed axially by seal cups 160, 162 against collar 202. O-rings 166, 168 are therefore energized, and press outwardly against the seal cups 160 162 even when the valve 10 is at rest and no external pressures are exerted thereon, albeit less so than when the supply or function pressures act thereon. The seals used in bi-directional seal assemblies—as well as in other structures—are often used as energizers. Therefore, desirable spring characteristics in such seals are important, and it would be beneficial to use seals with better spring characteristics than that of standard O-rings.